Aphids, natural enemies and smarter management

 

 

 

 

 

Aphids are soft-bodied insects with piercing and sucking mouthparts, which they use to penetrate plant tissue and extract sap. They have complex lifecycles, including both sexual and asexual reproduction, and the production of winged (alate) or wingless (apterous) adult forms. In the warm Australian climate, the sexual phase is reduced and reproduction is almost entirely asexual, with females giving birth to live female offspring that are already carrying development embryos. This ‘telescoping’ of generations allows aphids to reproduce very rapidly when conditions are favourable. Aphid survival, development and population growth are strongly influenced by local environmental factors; cooler temperatures or significant rainfall slow rates of aphid development while warm and relatively dry conditions favour rapid population build-up.

 

 

Aphid populations survive over summer on alternative host plants such as weeds in roadside vegetation and verges. In autumn, winged adults move into crop edges where they build up in numbers before moving into other parts of the crops. The timing and number of aphid flights into crops are influenced by late summer- and early autumn- rainfall; good rainfall favours growth of suitable host plants and allows aphids to build up early in the season. During the cool winter months, rates of aphid development are slow. Spring triggers a rapid increase in rates of aphid development, when reproduction and population expansion occurs very quickly. Aphids often form dense colonies on the growing points of individual plants before moving onto surrounding plants; therefore aphid infestations often occur in patches or ‘hotspots’ within paddocks.

 

 

Aphids damage crop plants directly through feeding which removes plant sap and nutrients, leading to wilting and yellowing of plants, and through secretion of honeydew, which supports secondary growth of sooty mould. Aphids also cause indirect crop damage through the transmission of plant virus diseases. Direct feeding damage usually only occurs when aphids are in high populations, however substantial plant damage can result from plant viruses transmitted by relatively few aphids. Aphids transmit viruses by feeding (or probing) on infected plants and soon after, feeding (or probing) healthy plants. Viruses transmitted non-persistently are carried temporarily on the stylets and will transmit (instantly) to only the next plant that is probed. Persistent viruses remain in the salivary glands of the aphid for life, which can transmit the virus to multiple plants. Acquisition of persistent viruses by aphids requires 1-2 hours of active feeding on infected plants, followed by a latent non-infective period of 12-24 hours. Transmission to healthy plants occurs after 5-10 minutes of active feeding.

 

 

 

There are a number of aphid species that attack field crops in Australia (Table 1). Aphids have a pair of wax-secreting tube-like projections called ‘siphuncles’ (or ‘cornicles’) at the rear of the abdomen. These often vary in size and shape and can be a characteristic feature for distinguishing between species. Species identification in the field is usually important because management strategies will vary. Only certain species will transmit plant viruses to particular crop types. Some species have also developed resistance to multiple chemical classes; hence selection of insecticides for controlling these species must be carefully considered. Because aphid species are generally crop-specific, correctly identifying aphids (particularly those on weeds and volunteer hosts) can also be used to predict the level of pest problems likely to be encountered on any given crop.

 

 

 

 

 

 

 

 

 

Monitoring and early detection of crop aphids is a key to effective management. Regular monitoring of most crops should commence in late winter and early spring, and continue through until late flowering. Standard recommendations for aphid monitoring are to check representative parts of entire paddocks along with any patches of wilted or stunted growth. Inspections should include at least five points within each paddock and look for aphids on a minimum of 20 plants at each point.

 

Economic thresholds (ETs) relate to the density of a given pest at which the cost of potential crop damage equals the costs of treatment (chemical + application costs), and provide a guide for growers to decide whether to implement control. ETs are quantitative measures and usually specified as the number of pests found per unit of crop area using a specified (standard) sampling technique. Yield loss and quality reduction are usually the critical factors governing control decisions. Unfortunately the lack of entomological broad-acre research in the southern grain belt over the last couple of decades has seen many ETs become outdated and somewhat irrelevant to current economic costs and management practices.

 

For aphids, there are some threshold recommendations available for Australian field crops that are based on research conducted in Western Australia (feeding damage only; in the absence of viruses). For cereals, consider control if 50% of plants have at least 15 aphids per tiller, for crops expected to yield at least 3 tonnes/ha. For canola, consider control if at least 20% of plants are infested. For lupins, control measures should be implemented if more than 30% of growing tips are colonised by aphids from the flower bud stage through to podding, particularly in aphid-susceptible varieties.

 

It is important to realise these thresholds are a guideline only and will vary considerably with agronomic factors such as crop growth stage, yield potential and vigour, as well as local weather conditions and moisture levels, and economic factors such as market value (grain prices), and chemical and application costs. Slightly moisture-stressed plants are often less able to compensate for aphid feeding damage compared with healthy plants. These thresholds need constant revision and updating to reflect changes in these factors over time and between regions.

 

 

Insecticides should be chosen carefully and used strategically for effective management of aphids. It is often not necessary to treat entire paddocks as aphid infestations often start at crop edges. If aphids are detected early, often a border spray with an aphid-specific insecticide (e.g. pirimicarb) will provide good control of aphids whilst minimising harm to beneficial insects within the crop. The use of synthetic pyrethroids is not recommended as it is largely ineffective in directly killing aphids (anti-feedant properties only), selects strongly for insecticide resistance in aphids and non-target species, and will kill many natural enemies. The use of some insecticide-based seed dressings may prevent aphids from entering crops for up to six weeks after emergence, delaying the need for foliar sprays.

 

The importance of correctly timing insecticide applications to control aphids should not be under-estimated. If combating viruses and applying insecticides in autumn, sprays should be applied soon after aphids start moving into crops from surrounding vegetation. Waiting too long could result in a substantial number of plants becoming infected, which is likely to result in larger issues with viruses as temperatures increase in spring. Remember that the application of broad-spectrum insecticides early may result in a later reinfestation because small numbers of surviving aphids are no longer controlled by natural enemies. Timing of insecticide applications in spring is also critical. Spraying too early (before significant aphid ‘flights’ have occurred) can be a waste of product and disrupt natural enemies present. However, aphid populations can reach damaging levels very rapidly under ideal conditions; therefore, if spraying is required, it is important to apply before this time (see Figure 1).

 

 

 

In virus-prone areas, monitoring should commence earlier in the autumn, as early viral infection can be most damaging. Monitor crops from emergence and weekly afterwards for signs of aphid colonisation, particularly in crop edges. The use of yellow sticky traps or pan traps can assist in early detection of aphids and assessment of numbers. Prevent aphid build-up in nearby weeds and other vegetation ‘green bridges’ to lessen the risk of nearby flights. Consider the use of seed dressings to prevent aphid building up in emerging crops early in the season.

 

 

 

 

 

 

 

 

 

 

 

 

 

Bray T. Virus control and aphid monitoring simplified. Australia Pulse Bulletin.

 

Edwards O, Franzmann B, Thackray D, Micic S, (2008). Insecticide resistance and implications for future aphid management in Australian grains and pastures: a review. Australian Journal of Experimental Agriculture 48, 1523-1530.

 

Schwinghamer M, Knights T, Moore K. Virus control in chickpeas – special considerations. Australia Pulse Bulletin.

 

PestFacts South-Eastern

http://cesarconsultants.com.au/services/agriculture.html

 

PestFacts SA and western Victoria

http://www.sardi.sa.gov.au/pestfacts

 

Crop insects: The Ute Guide (Southern Grain belt edition)

http://www.grdc.com.au/director/events/bookshop?pageNumber=2&category=204

 

 

 

Kym Perry Ph: 08 8303 9370

Email: kym.perry@sa.gov.au

 

Ken Henry Ph: 08 8303 9540

Email: ken.henry@sa.gov.au

 

Judy Bellati Ph: 08 8303 9670

Email: judy.bellati@sa.gov.au

 

Paul Umina Ph: 03 8344 2522

Email: pumina@unimelb.edu.au

 

Stuart McColl Ph: 03 9329 8816

Email: stuart@cesarconsultants.com.au